Goods taxed in earlier regime returned in GST regime

Here is a situation :
I have, on customers assent, sent goods to a warehouse, outside the state, booked as sale (outward supply ) .Invoice is raised under Excise and CST law .
Now the customer has refused to recieve the goods and it is lying in the interstate warehouse .
1. How can I receive back the goods without paying tax (as it is not sales return and it is a burden to pay tax on a supply not intended to be made by me ) ?
2. Even if I get the goods back is there any way to claim refund or credit of excise and CST which was charged earlier and now will be reversed ?

Awaiting answers .